Stacked packaging improvements

ABSTRACT

A plurality of microelectronic assemblies are made by severing an in-process unit including an upper substrate and lower substrate with microelectronic elements disposed between the substrates. In a further embodiment, a lead frame is joined to a substrate so that the leads project from this substrate. Lead frame is joined to a further substrate with one or more microelectronic elements disposed between the substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/666,975, filed on Aug. 25, 2008 which is a national phase entry under35 U.S.C. §371 of International Application No. PCT/US2005/039716 filedNov. 3, 2005, published in English, which claims the benefit of U.S.Provisional Patent Application No. 60/624,667, filed Nov. 3, 2004. Thedisclosures of the aforesaid applications are hereby incorporated byreference herein.

BACKGROUND OF THE INVENTION

Microelectronic elements such as semiconductor chips typically areprovided in packages which provide physical and chemical protection forthe semiconductor chip or other microelectronic element. Such a packagetypically includes a package substrate such as a small circuit panelformed from a dielectric material and having electrically conductiveterminals thereon. The chip is mounted on the panel and electricallyconnected to the terminals of the package substrate. Typically, the chipand portions of the substrate are covered by an encapsulant orovermolding, so that only the terminal-bearing outer surface of thesubstrate remains exposed. Such a package can be readily shipped, storedand handled. The package can be mounted to a larger circuit panel suchas a circuit board using standard mounting techniques, most typicallysurface-mounting techniques. Considerable effort has been devoted in theart to making such packages smaller, so that the packaged chip occupiesa smaller area on the circuit board. For example, packages referred toas chip-scale packages occupy an area of the circuit board equal to thearea of the chip itself, or only slightly larger than the area of thechip itself. However, even with chip-scale packages, the aggregate areaoccupied by several packaged chips is greater than or equal to theaggregate area of the individual chips.

It has been proposed to provide “stacked” packages, in which a pluralityof chips are mounted one above the other in a common package. Thiscommon package can be mounted on an area of the circuit panel which maybe equal to or just slightly larger than the area typically required tomount a single package containing a single chip. The stacked packageapproach conserves space on the circuit panel. Chips or other elementswhich are functionally related to one another can be provided in acommon stacked package. The package may incorporate interconnectionsbetween these elements. Thus, the main circuit panel to which thepackage is mounted need not include the conductors and other elementsrequired for these interconnections. This, in turn, allows use of asimpler circuit panel and, in some cases, allows the use of a circuitpanel having fewer layers of metallic connections, thereby materiallyreducing the cost of the circuit panel. Moreover, the interconnectionswithin a stacked package often can be made with lower electricalimpedance and shorter signal propagation delay times than comparableinterconnections between individual packages mounted on a circuit panel.This, in turn, can increase the speed of operation of themicroelectronic elements within the stacked package as, for example, byallowing the use of higher clock speeds in signal transmissions betweenthese elements.

One form of stacked package which has been proposed heretofore issometimes referred to as a “ball stack.” A ball stack package includestwo or more individual units. Each unit incorporates a unit substratesimilar to the package substrate of an individual package, and one ormore microelectronic elements mounted to the unit substrate andconnected to the terminals on the unit substrate. The individual unitsare stacked one above the other, with the terminals on each individualunit substrate being connected to terminals on another unit substrate byelectrically conductive elements such as solder balls or pins. Theterminals of the bottom unit substrate may constitute the terminals ofthe package or, alternatively, an additional substrate may be mounted atthe bottom of the package and may have terminals connected to theterminals of the various unit substrates. Ball stack packages aredepicted, for example, in certain preferred embodiments of U.S.Published Patent Applications 2003/0107118 and 2004/0031972, thedisclosures of which are hereby incorporated by reference herein.

In another type of stack package sometimes referred to as a fold stackpackage, two or more chips or other microelectronic elements are mountedto a single substrate. This single substrate typically has electricalconductors extending along the substrate to connect the microelectronicelements mounted on the substrate with one another. The same substratealso has electrically conductive terminals which are connected to one orboth of the microelectronic elements mounted on the substrate. Thesubstrate is folded over on itself so that a microelectronic element onone portion lies over a microelectronic element on another portion, andso that the terminals of the package substrate are exposed at the bottomof the folded package for mounting the package to a circuit panel. Incertain variants of the fold package, one or more of the microelectronicelements is attached to the substrate after the substrate has beenfolded to its final configuration. Examples of fold stacks are shown incertain preferred embodiments of U.S. Pat. No. 6,121,676; U.S. patentapplication Ser. No. 10/077,388; U.S. patent application Ser. No.10/655,952; U.S. Provisional Patent Application No. 60/403,939; U.S.Provisional Patent Application No. 60/408,664; and U.S. ProvisionalPatent Application No. 60/408,644. Fold stacks have been used for avariety of purposes, but have found particular application in packagingchips which must communicate with one another as, for example, informing assemblies incorporating a baseband signal processing chip andradiofrequency power amplifier (“RFPA”) chip in a cellular telephone, soas to form a compact, self-contained assembly.

Despite all of these efforts in the art, still further improvement wouldbe desirable. In particular, it would be desirable to provide packageswhich can afford advantages similar to those achieved in a fold stackwithout the necessity for actually folding a substrate.

SUMMARY OF THE INVENTION

One aspect of the invention provides a method of making a plurality ofmicroelectronic assemblies. The method according to this aspect of theinvention desirably includes the steps of providing an in-process unitincluding a plurality of microelectronic elements, a least one uppersubstrate extending above the microelectronic elements and at least onelower substrate extending below the microelectronic elements, at leastone of these substrates including a plurality of regions; and thensevering the in-process unit to form individual units, each said unitincluding a region of each of said at least one of said substrates andat least one of said microelectronic elements.

A further aspect of the invention provides an in-process unit. Thein-process unit according to this aspect of the invention desirablyincludes upper and lower substrates and a plurality of microelectronicelements disposed between the substrates. Each substrate preferablyincludes a plurality of regions, each region of the upper substratebeing aligned with a corresponding region of the lower substrate with atleast one said microelectronic element disposed therebetween.Preferably, each of the regions of said upper and lower substrates haveelectrically conductive elements, at least some of said conductiveelements of each region of the upper substrate being electricallyconnected to electrically conductive elements of the correspondingregion of said lower substrate.

Yet another aspect of the invention provides a method of making amicroelectronic assembly. The method according to this aspect of theinvention desirably includes attaching a lead frame to a first substrateso that leads of the lead frame project from such substrate andassembling the first substrate with a second substrate so that at leastone microelectronic element is disposed between the first and secondsubstrates, and connecting said leads to said second substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic sectional view depicting elements utilized in aprocess according to one embodiment of the invention.

FIGS. 2-6 are views similar to FIG. 1, but depicting the elements atprogressively later stages of the same process.

FIG. 7 is a diagrammatic sectional view of elements used in a processaccording to a further embodiment of the invention.

FIG. 8 is a diagrammatic section view depicting elements used in aprocess according to yet another embodiment of the invention.

FIG. 9 is a view similar to FIGS. 7 and 8, but depicting elements usedin a process according to a further embodiment of the invention.

FIG. 10 is a view depicting the elements shown in FIG. 9 at a laterstage of the process.

FIG. 11 is a diagrammatic view depicting a substrate and lead frameutilized in a process according to a further embodiment of theinvention.

FIG. 12 is a view depicting the substrate and lead frame of FIG. 11 at alater stage in the process.

FIG. 13 is a diagrammatic sectional view depicting the elements shown inFIGS. 11 and 12 during a still later stage of the process.

FIG. 14 is a diagrammatic sectional view depicting an assembly madeusing the elements of FIGS. 11-13.

DETAILED DESCRIPTION

An assembly method in accordance with one embodiment of the inventionutilizes a substrate referred to herein for purposes of convenience asthe lower substrate 20 incorporating a dielectric layer 21 defining anupper surface 22 and a lower surface 24. The lower substrate 20typically is in the form of a continuous or semicontinuous tape or sheethaving a large number of regions 26. As explained below, each region 26will constitute a portion of an individual package at the end of theprocess, and each region 26 includes the features which, as discussedbelow, will form a part of a single package.

Dielectric layer 21 may be a single layer, or may be a laminateincluding several sublayers. The dielectric layer desirably is formedprimarily from polymeric dielectrics such as polyimide, BT resin, epoxyor other dielectric polymers, and may include reinforcing fibers as, forexample, glass fibers. Dielectric layer 21 may be flexible or rigid.Lower substrate 20 includes mounting terminals 28, and laterinterconnect terminals 29, exposed at the lower surface 24 of thedielectric layer and conductive connection elements 30 exposed at theupper surface 22. In the particular embodiment depicted, terminals 28and 29 are formed in a layer separate from connection elements 30, theselayers being separated from one another by dielectric layer 21 andelectrically connected to one another by conductive elements such asvias 32 extending through the dielectric layer. Such an arrangement iscommonly referred to as a “two-metal” structure. However, lowersubstrate 20 can be formed as a single metal structure with a singlemetal layer constituting conductive connection elements 30 as well asterminals 28 and 29. For example, such a layer may be disposed on thebottom surface 24 of the dielectric layer, with the conductiveconnection elements 30 exposed at the top surface 22 through holes (notshown) in the dielectric layer. Similarly, such a single metal layer maybe disposed on the upper surface 22, with the terminals 28 and 29 beingexposed at the lower surface 24 through holes (not shown) in thedielectric layer. In still further alternatives, one or more metalliclayers constituting the conductive mounting elements, the terminals orboth can be disposed within the thickness of the dielectric layer andexposed through holes to the appropriate surfaces.

Lower substrate 20 has apertures 34 extending through the dielectriclayer, from the upper surface to the lower surface. Apertures 34 may bein the form of individual holes or elongated slots. Apertures 34 aredisposed in the vicinity of interlayer connection terminals 29.Microelectronic elements 36 are mounted on the upper surface 22 of lowersubstrate 20. Each region 26 has one or more of the microelectronicelements mounted thereon. In the particular embodiment illustrate, eachregion 26 of the lower substrate bears one microelectronic element. Themicroelectronic elements shown are semiconductor chips mounted in aface-down orientation, with the contacts (not shown) of the chipconnected to the conductive connection elements 30 of the substrate as,for example, by bonding the contacts to the conductive mounting elementsusing a bonding material such as a solder. However, other techniques canbe employed. For example, each microelectronic element 36 may be apackaged microelectronic element incorporating a package substrate (notshown) with terminals thereon, these terminals being connected to theconductive connection elements 30 on the lower substrate. In still othervariants, techniques such as anisotropic conductive adhesives can beemployed. An overmolding 38 covers the exposed surfaces of eachmicroelectronic element 36. In other embodiments, overmolding 38 isomitted. The microelectronic element 36 within each region 26 of thelower substrate is electrically connected through the conductiveconnection elements 30 of that region to at least some of the mountingterminals 28 of the same region, to at least some of the interlayerconnection terminals 29 of that region or both. Microelectronic elements36 may be mounted on the lower substrate using conventional techniques,either as part of the assembly process described herein or in a separateoperation used to prepare the lower substrate 20.

The process according to this embodiment of the invention also uses anupper substrate 40 including a dielectric layer 41, which may be formedfrom the same materials as discussed above in connection with the lowerdielectric layer, defining an upper surface 42 and a lower surface 44.The upper substrate has layer interlayer connection terminals 49 exposedat lower surface 44, and conductive mounting terminals 50 exposed at theupper surface. Here again, these features are shown as a two-layerstructure, but can be formed from a single layer or multiple layers withthe features exposed to one or both of the surfaces through holes in thedielectric layer. The upper substrate 40 also has a plurality of regions46, each such region including a set of interlayer connection terminals49 and a set of mounting terminals 50, at least some mounting terminals50 electrically connected to at least some interlayer connectionterminals 49 of the same region.

In the assembly process, lower substrate 20 with microelectronicelements 36 thereon is united with upper substrate 40, so that the lowersurface 44 of the upper substrate 40 rests on the microelectronicelements 36 and faces toward the lower substrate. Thus, themicroelectronic elements 36 are positioned between the substrates. Anadhesive 52 may be applied on the lower surface 44 of the uppersubstrate on the surfaces of microelectronic elements 36 remote from thelower substrate, which surfaces may be the surfaces defined by theencapsulant 38 surrounding each microelectronic element. The process ofassembling the substrates to one another most preferably is conductedwhile both substrates remain in the form of large substratesincorporating plural regions 26 and 46. For example, where thesubstrates are in the form of elongated tapes or strips, the substratesmay be advanced through a pair of nip rollers or through a press, so asto bring the upper substrate into engagement with the surfaces ofmicroelectronic elements 36 on the lower substrate. Alternatively, whereboth substrates are in the form of large sheets, such as large circularor square sheets, the assembly process may be conducted simply by layingone sheet onto the other sheet, so as to assemble the substrate with oneanother. The substrates are assembled with one another so that eachregion 46 of the upper substrate 20 is aligned with a correspondingregion 26 of the lower substrate 20.

After assembling the substrates with one another, the layer interconnectterminals 29 in each region of the lower substrate are connected to thelayer interconnect terminals 49 of the corresponding region on the uppersubstrate. This connection is made by applying wire bonds 53 between thelayer interconnect terminals. The wire bonds extend through theapertures 34 in the lower substrate. After wire-bonding, at least someof the lower mounting terminals 28, or at least some contacts on thechip 36 associated with each lower region, are connected to at leastsome of the mounting terminals 50 on the corresponding region of theupper substrate through the wire bonds and layer interconnect terminals.

Following application of the wire bonds, an encapsulant 54 is introducedbetween the lower substrate 20 and upper substrate 40 (FIG. 4). Theencapsulant may be any flowable encapsulant compatible with thematerials of construction. Most desirably, the encapsulant 54 is asettable material which, in an uncured state, is a liquid having arelatively low viscosity, and which can be cured to a solid or semisolidcondition. Examples of such materials include epoxies, silicones andother materials commonly employed as encapsulants in microelectronicpackages. These materials cure by chemical reaction, typically promotedby application of heat. Other encapsulants such as thermoplasticmaterials which liquify upon heating, and cure to a solid condition bycooling, can be used. The encapsulant can be injected between thesubstrates by any suitable process. During injection of the encapsulant,some encapsulant may escape through the apertures 34 (FIG. 3) in thelower substrate. The substrates may be constrained between elements of amold or other fixture during injection of the encapsulant, and theseelements may seal the openings 34 in the upper substrate. Alternativelyor additionally, the openings 34 in the lower substrate may be coveredby dielectric film such as a solder mask applied over the openings afterwire-bonding. The techniques taught in commonly assigned U.S. Pat. Nos.6,329,224 and 5,766,987, the disclosures of which are herebyincorporated by reference herein, may be employed in this step. Theencapsulant injection step desirably is also performed while thesubstrates 40 and 20 remain in their original form, with the variousregions of each substrate remaining connected to one another at thisstage. The encapsulant surrounds the wire bonds 53 (FIG. 3) anddesirably substantially or completely fills the space between the upperand lower substrate, other than the spaces occupied by themicroelectronic elements themselves.

After injection and curing of the encapsulant, one or more additionalmicroelectronic elements 56 are mounted on the exposed top surface 42 ofupper substrate 40, and electrically connected to the mounting terminals50 of the upper substrate. Here again, the microelectronic elements 56are mounted to the various regions 46 of the upper substrate.Electrically conductive bonding materials such as solder balls 58 may beapplied on the mounting terminals 28 of the lower substrate. Theadditional microelectronic elements 56 may be “bare” or unpackagedsemiconductor chips or other microelectronic elements, or may bepackaged microelectronic elements such as packaged semiconductor chips.In the embodiments depicted, each additional microelectronic element ismounted by directly bonding contacts on the microelectronic element tothe mounting elements 50 of the upper substrate. However, other mountingand connection techniques can be used. For example, in a variant, theadditional microelectronic element 56 may be mounted in a “face-up”disposition on the upper substrate and connected by wire bonds to themounting elements 50. Also, an encapsulant or other cover may be appliedover the additional microelectronic elements.

After mounting the additional microelectronic elements 56 and theconductive bonding materials 58, the upper and lower substrates aresevered to form individual units 60 (FIG. 6). Each such unit includesone region 26 of the lower substrate and the corresponding region 46 ofthe upper substrate, together with the microelectronic 36 on the lowersubstrate and additional microelectronic element 56 on the uppersubstrate. Each such unit is a self-contained stacked package. Each unit60 forms a complete stacked package, with one or more additionalmicroelectronic elements 56 connected to one or more microelectronicelements 36. Such a package can be mounted on a circuit board or otherlarger substrate in substantially the same way as a conventional singleelement microelectronic package.

In a variant of the process discussed above, the additionalmicroelectronic elements 56, connective bonding materials 58 or both canbe mounted to the substrates after severance. The assembled substratesor microelectronic elements 36, with or without the bonding materials58, in either the unsevered condition or as separate, severed units, canbe handled, shipped and stocked as semifinished articles of commerce.Such an arrangement can be used, for example, where the samemicroelectronic elements 36 are to be incorporated into a large numberof packages, but different additional elements 56 are used in differentones of the packages.

In yet another variant, the encapsulant 54 may be omitted. In thisvariant, the microelectronic elements 36 disposed between the substratesprovide structural support. Additional structural support may beprovided between the substrates by providing spacers extending betweenthe dielectric elements at locations not occupied by microelectronicelements 36 or wire bonds 53.

A process according to a further embodiment of the invention uses alower substrate 120 and upper substrate 140 similar to those discussedabove with reference to FIGS. 1-6. However, the microelectronic elements136 mounted on lower substrate 120 are mounted in “face-up” disposition,without overmolding. The contacts on the microelectronic elements 136are electrically connected to the conductive mounting elements 130 onthe upper surface of lower substrate 120 by wire bonds 102 beforeassembly of the upper substrate 140. Spacers 104 are provided on theupwardly facing surfaces of microelectronic elements 136 or on the lowerface of upper substrate 140, so as to hold the upper substrate abovewire bonds 102. Spacers 104 desirably are formed from a dielectricmaterial, and may include or consist of an adhesive layer. Here again,the interlayer connection terminals 129 of the lower substrate areconnected to the interlayer connection pads 149 of the upper substrateby wire bonds 152. After wire-bonding, the assembly shown in FIG. 7 canbe processed and handled in the same manner as discussed above withreference to FIGS. 4-6.

A process according to yet another embodiment of the invention againutilizes a lower substrate 220 and upper substrate 240 similar to thosediscussed above. Microelectronic elements 236 are mounted on the uppersurface 222 of the lower substrate 220. Desirably, these microelectronicelements are covered by overmolding 238 around each microelectronicelement. Here again, the microelectronic elements 236 may be packaged orunpacked elements. However, in the embodiment of FIG. 8, the interlayerconnection terminals 229 of the lower substrate are exposed at the uppersurface 222 of the substrate, whereas the interlayer connectionterminals 249 of the upper substrate are exposed at the lower surface244 of the upper substrate. These substrates are assembled with oneanother in a manner similar to that discussed above. However,electrically conductive spacing elements such as solder balls arepositioned between the substrates on interlayer connection terminals,229 of the lower substrate or 249 of the upper substrate. When thesubstrates are assembled with one another, the conductive elementsengage the interlayer connection terminals on the opposite substrate andare bonded thereto. Conductive elements 202, thus, provide bothelectrical connection between the substrates and physical spacingbetween the substrates. Additional microelectronic elements 256 may bemounted on the upper substrate before or after assembly. As in the otherembodiments discussed above, the assembly steps serves to interconnectnumerous regions of the upper substrate with numerous regions of thelower substrate in a single operation. As in the embodiments discussedabove, the interconnected substrates can be severed so as to formindividual units. An encapsulant (not shown) optionally may be injectedbetween the substrates in the manner discussed above, desirably beforesevering the substrates. In a further variant (FIG. 9), themicroelectronic elements 336 on the lower substrate 320 areunencapsulated “bare” semiconductor chips. These chips are wire-bondedto the conductive mounting components 330 of the lower substrate usingwire bonds 302 similar to the wire bonds discussed above with referenceto FIG. 7. The upper substrate 340 is assembled to the lower substrateand connected to the lower substrate by conductive elements 304 similarto those discussed above with reference to FIG. 8. Desirably, anencapsulant 354 (FIG. 10) is injected between the substrates prior tosevering the substrates to form individual units. Conductive elementsother than solder balls may be employed in the various embodiments. Forexample, as disclosed on PCT Published International Patent ApplicationWO 2004/077525, the disclosure of which is hereby incorporated byreference herein, metallic conductive elements in the form of elongatedbumps or pins may be used as inter-unit connections in a stack package.As set forth in U.S. Provisional Patent Application No. 60/583,066,filed Jun. 25, 2004, the disclosure of which is also incorporated byreference herein, pins of the types disclosed in co-pending, commonlyassigned U.S. Provisional Patent Applications 60/533,210; 60/533,393 and60/533,437, all filed Dec. 30, 2003, the disclosures of which are allhereby incorporated by reference herein, can be used as inter-unitconnections in a stack package. Pins of these and other types can beused in the assemblies discussed above. One or both of the substratesmay be provided with these pins prior to assembly, so that the pins areengaged with interlayer connection terminals on the opposite substrate.

A process according to yet another embodiment of the invention utilizesan upper substrate 440 in the form of a single metal tape incorporatinga dielectric layer 421 with upper mounting terminals 450 and interlayerconnection terminals 449 defined by a single layer of metallic featureson the lower surface of the tape, the mounting 450 being exposed throughholes 451 in the dielectric layer to upper surface 422 of the uppersubstrate. A lead frame including numerous leads 452 is attached toupper substrate 440 so that each lead 452 extends from one of theinterlayer connection terminals 449, as seen in FIG. 12. Although onlytwo leads 452 are depicted in the drawings, it should be appreciatedthat the lead frame includes numerous leads, and may also include a busbar or other elements to hold the leads in position relative to oneanother. Bus bars or other retaining elements may be removed afterassembly of the lead frame with the upper substrate. A lead frame of thetype taught in co-pending, commonly assigned U.S. patent applicationSer. No. 10/746,810, filed Dec. 24, 2003, the disclosure of which ishereby incorporated by reference herein, may be utilized. The lead framemay be bonded to the interlayer connection terminals 449 of the uppersubstrate by processes such as solder-bonding, diffusion-bonding,thermocompression-bonding or the like. Alternatively, interlayerconnection terminals 449 may be made in the form of tape-automatedbonding (“TAB”) leads, and these leads may be bonded to the lead frameusing processes similar to those commonly used to bond TAB leads toelements such as semiconductor chips. As best seen in FIG. 12, the leads452 of the lead frame project downwardly from the upper substrate 440.The process also utilizes a lower substrate 420 which has lower mountingterminals 428 exposed at its lower surface 424, and has electricalconnections 430 exposed at its upper surface and interlayer connectionterminals 429 also exposed at its upper surface 422. Here again, in theparticular embodiment depicted in FIG. 13, the lower substrate is shownas a “two-metal” structure, but could be a single metal structure withvarious features exposed through holes in the dielectric element 421 ofthe lower substrate. A semiconductor chip or other microelectronicelement 436 is mounted to lower substrate 420. In the embodimentdepicted, the semiconductor chip 436 is mounted in face-up dispositionand connected by wire bonds 402 to the connection terminals 430.However, chip 436 could also be mounted face-down. In a further variant,chip 436 could be a packaged chip or other packaged microelectronicelement. In the particular embodiment depicted in FIG. 13, chip 436 issupported above the dielectric element 421 of the lower substrate by aspacer 404. In a further variant, spacer 404 could be replaced by afurther semiconductor chip or other microelectronic element which may bemounted face-up or face-down. A spacer 406, desirably formed from adielectric material, is disposed on the surface of microelectronicelement 436 remote from lower substrate 420.

The subassembly including the upper substrate 440 and leads 452 of thelead frame is mounted to the lower substrate by advancing thesubassembly toward the lower substrate and bonding the lower ends ofleads 452, remote from upper substrate 441 to the interlayer connectionterminals 429 of the lower substrate using any of the techniquesdiscussed above. After assembly of the upper and lower substrates, theresulting unit, including lower substrate 420, upper substrate 440,microelectronic element 436 and leads 452 connecting the upper and lowersubstrates, is encapsulated as, for example, by introducing a flowableencapsulant around the microelectronic element 436 and betweensubstrates 420 and 440. The encapsulation process is conducted so as toleave upper mounting terminals 450 and lower mounting terminals 428exposed and uncovered by the encapsulant 454. All of the steps discussedabove with reference to FIGS. 11-14 may be conducted using individualupper and lower substrates and/or individual lead frames, or may beconducted while the upper substrate, the lower substrate, the leadframes or any combination of these are in the form of larger assembliessuch as tapes or strips incorporating numerous substrates and/ornumerous lead frames. In this case, the larger elements are severed asdiscussed above, so as to form individual units, each including a lowersubstrate, an upper substrate and one or more microelectronic elements436. Here again, the larger units, before severance, can be handled,shipped and stocked as an article of commerce. Also, the individualunits can be handled as such. Here again, a packaged or unpackagedadditional microelectronic element 456 may be mounted to the uppermounting terminal as, for example, by solder-bonding, as seen in FIG.14, or by wire-bonding. The lower mounting terminals 428 may be providedwith electrically conductive bonding material such as solder balls 408and may be used to mount the finished assembly to a circuit panel.

In each of the embodiments discussed above, the roles of the upper andlower substrates may be reversed. For example, the upper mountingterminals 450 of the assembly seen in FIG. 14 may be used to mount theassembly to a circuit panel, whereas the lower mounting terminals 428may be used to mount a further microelectronic element to the assembly.Also, the leads 452 of the lead frame may be assembled to the lowersubstrate rather than the upper substrate. In yet a further embodiment,the entire upper substrate may consist solely of elements from a leadframe. The bus bars or other parts of the lead frame which serve tointerconnect the various leads and form a self-supporting lead frame maybe removed after encapsulation. Conversely, the lower substrate 420 maybe replaced by elements of the lead frame. In one variant, the ends ofthe lead frame remote from the upper substrate are exposed so that theseends serve as the lower mounting terminals of the assembly.

As used in this disclosure, terms such as “upper,” “lower,” “upwardly”and “downwardly,” and similar terms denoting directions, refer to theframe of reference of the components themselves, rather than to thegravitational frame of reference. With the parts oriented in thegravitational frame of reference in the directions shown in the figures,with the top of drawing being up and the bottom of the drawing beingdown in the gravitational frame of reference, the upper substrate is,indeed, above the lower substrate in the gravitational frame ofreference. However, when the parts are turned over, with the top of thedrawing facing downwardly in the gravitational frame of reference, theupper substrate is below the lower substrate in the gravitational frameof reference.

The foregoing descriptions of the preferred embodiments are intended toillustrate rather than to limit the present invention.

As these and other variations and combinations of the features discussedabove can be utilized without departing from the present invention asdefined by the claims, the foregoing description of the preferredembodiments should be taken by way of illustration rather than by way oflimitation of the invention as defined by the claims.

1. A microelectronic unit, comprising: an upper dielectric elementhaving first electrically conductive elements thereon; a lowerdielectric element having second electrically conductive elementsthereon; a microelectronic element disposed between said upper and lowerdielectric elements; and a plurality of wire bonds having first endsjoined to said first conductive elements and second ends remote fromsaid first ends joined to said second conductive elements andelectrically connecting said upper and lower dielectric elements.
 2. Themicroelectronic unit as claimed in claim 1, further comprising anencapsulant disposed at least between said upper and lower dielectricelements.
 3. The microelectronic unit as claimed in claim 2, whereinsaid encapsulant overlies said microelectronic element and at leastsubstantially fills a volume between said upper and lower dielectricelements.
 4. The microelectronic unit as claimed in claim 1, wherein atleast one of said wire bonds extends beyond a peripheral edge of atleast one of said dielectric elements.
 5. The microelectronic unit asclaimed in claim 1, wherein at least one of said dielectric elements hasan aperture, and at least one of said wire bonds extends through saidaperture.
 6. The microelectronic unit as claimed in claim 1, wherein theupper dielectric element has an upper surface facing away from themicroelectronic element and upper terminals exposed at said uppersurface, at least one of said upper terminals overlies a first majorsurface of said microelectronic element, and the lower dielectricelement has a lower surface facing away from said microelectronicelement and lower terminals exposed at said lower surface, wherein atleast one of said lower terminals overlies a second major surface ofsaid microelectronic element remote from said first major surface, saidat least one upper and lower terminals being electricallyinterconnected.
 7. The microelectronic units as claimed in claim 1,wherein at least some of the upper and lower terminals are electricallyconnected with said first and second conductive elements.
 8. An assemblyincluding the microelectronic unit claimed in claim 6, furthercomprising at least one microelectronic element overlying said uppersurface and electrically interconnected with said at least one terminal.9. A microelectronic unit, comprising: an upper dielectric elementhaving first electrically conductive elements thereon; a lowerdielectric element having second electrically conductive elementsthereon; a microelectronic element disposed between said upper and lowerdielectric elements; and a plurality of electrically conductive spacingelements joined to said respective first and second conductive elementsand electrically connecting said upper and lower dielectric elements,said electrically conductive spacing elements defining a spacing betweensaid upper and lower substrates.
 10. The microelectronic unit as claimedin claim 9, wherein said electrically conductive spacing elements aresolder balls.
 11. The microelectronic unit as claimed in claim 9,further comprising an encapsulant disposed at least between said upperand lower dielectric elements.
 12. The microelectronic unit as claimedin claim 11, wherein said encapsulant overlies said microelectronicelement and at least substantially fills a volume between said upper andlower dielectric elements.
 13. The microelectronic unit as claimed inclaim 9, wherein the upper dielectric element has an upper surfacefacing away from the microelectronic element and upper terminals exposedat said upper surface, at least one of said upper terminals overlies afirst major surface of said microelectronic element, and the lowerdielectric element has a lower surface facing away from saidmicroelectronic element and lower terminals exposed at said lowersurface, wherein at least one of said lower terminals overlies a secondmajor surface of said microelectronic element remote from said firstmajor surface, said at least one upper and lower terminals beingelectrically interconnected.
 14. The microelectronic units as claimed inclaim 13, wherein at least some of the upper and lower terminals areelectrically connected with said first and second conductive elements.15. An assembly including the microelectronic unit claimed in claim 13,further comprising at least one microelectronic element overlying saidupper surface and electrically interconnected with said at least oneterminal.